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There are probably a lot of reasons why humans should not have sex with animals, and a new study adds cancer to the list.

In a study of 492 men from rural Brazil, 35 percent reprted having had sex with an animal at men who had sex some point in their lives. The report found those men were twice as likely to develop cancer of the penis.

The paper was authored by a team of urologists from centers around Brazil and studied men from age 18 to 80. Men who had had sex with animals also reported a higher incidence of sexually transmitted diseases. Of the 118 penile cancer patients, 45 percent reported having had sex with animals, compared to 32 percent of healthy men. Fifty-nine percent of men who had sex with animals did so for one to five years, while 21 percent continued the behavior, also known as zoophilia, for more than five years.

Men reported having sex as frequently as daily with such animals as mares, cows, pigs and and chickens. Penile cancer accounts for up to 10 percent of cancers in men in Asia, Africa and South America, although it is rare in the U.S.

"We think that the intense and long-term SWA practice could produce micro-traumas in the human penile tissue," lead author Stênio de Cássio Zequi, a urologist inSão Paulo, said. "The genital mucus membranes of animals could have different characteristics from human genitalia, and the animals' secretions are probably different from human fluids."

The authors also said it's "absolutely plausible" that microorganisms present during sex with animals could cause penile cancer, citing the fact that human papilloma virus (HPV) causes cervical and penile cancer.